This patent application claims benefit of and priority to Jap. Pat. Application No. P2000-298277 filed Sep. 29, 2000; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an infrared sensor and method of fabricating it and, more particularly, to a low-cost, high-sensitivity, uncooled infrared sensor and method of fabricating it.
2. Description of the Related Art
Infrared imaging makes it possible to image objects night and day. Also, infrared radiation has a feature that it has higher permeability into smoke and fog than visible light. In addition, infrared imaging can obtain information about the temperature of the subject. Therefore, infrared imaging finds extensive use as monitor cameras and fire detection cameras, as well as use in military defense applications.
Quantum-type solid-state infrared imagers as mainstream devices have a drawback that they must be operated at cryogenic temperatures and thus a cooling mechanism is necessary. In recent years, uncooled solid-state infrared imaging devices free of this drawback have been developed rigorously. In the uncooled, i.e., thermal type, solid-state infrared imager, incident infrared radiation having a wavelength of about 10 xcexcm is converted into heat by an absorption mechanism. This heat causes a change in the temperature of the heat-sensitive portion. This temperature change is converted into an electrical signal by a thermoelectric conversion, and the electrical signal is read out. In this way, infrared image information is obtained.
Methods for improving the sensitivity of such an uncooled infrared sensor are classified into the following three major categories.
One method for improving the sensitivity is to improve the ratio of the infrared power, dP, incident on the infrared detection portion to the variation, dTs, of the temperature of the target, i.e., dP/dTs. In this method, the sensitivity improvement is mainly achieved by optics. That is, an infrared lens having a larger diameter is used. An antireflective film is coated. A low-absorption lens material is used. The infrared absorptivity of the infrared detection portion is improved. The infrared absorption area is increased. As uncooled infrared sensors have been equipped with an increasing number of pixels in recent years, most unit cells have come to use a size of approximately 40 xcexcmxc3x9740 xcexcm. Of the aforementioned items, improvement of the IR absorption area of the infrared detection portion remains a relatively important issue. However, it was reported that the IR absorption area has been successfully improved up to about 90% of the pixel area by forming an IR absorption layer on top of the pixels (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999). It will be difficult to achieve a higher sensitivity improvement by optical improvements.
Another method for improving the sensitivity is to improve the ratio of the variation, dTd, of the temperature of the infrared detection portion to the power, dP, of the incident infrared radiation, i.e., dTd/dP. This method is a thermal method, while the method previously described is an optical procedure. Generally, in an uncooled infrared sensor mounted in a vacuum package, heat conduction via support structures for supporting the infrared detection portion above a hollow structure inside the support substrate is currently prevalent in the transportation of heat from the infrared detection portion to the support substrate. Accordingly, leglike support structures made of a material having a low coefficient of thermal conduction are laid out such that they are made as thin and long as the design permits (e.g., Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999).
An infrared sensor having leglike support structures is described. FIG. 22 is a cross-sectional view showing a cross-sectional structure of infrared detection pixels in the infrared sensor having the prior art support leg structures. As shown in this figure, an SOI (silicon-on-insulator) substrate is formed by a silicon substrate 506, a buried oxide film 508, and a single-crystal silicon film 509. An infrared detection portion is formed on the patterned single-crystal silicon film 509 on this SOI substrate. This infrared detection portion utilizes a silicon pn junction described later. The single-crystal silicon film 509 under the single-crystal silicon substrate 506 is partially etched away to form a hollow structure 507. A dielectric film 510 is formed on the silicon substrate 506. A laminate structure consisting of a reflective layer 501, a dielectric layer 502, and an infrared absorber layer 503 is formed on the single-crystal silicon film 509. Infrared radiation is absorbed and converted into heat in this laminate structure. The produced heat is transmitted to the infrared detection portion of the single-crystal silicon film 509. A temperature variation due to heat is converted into a voltage change. An electrical signal caused by the voltage change is transmitted to conductive interconnects 517 in peripheral circuitry via a conductive interconnect 516. In FIG. 22, support leg structures include the conductive interconnect 516 and the dielectric film 510 surrounding the leg structures, and support the single-crystal silicon film pattern 509 above the substrate.
While the pixel size has been reduced to about 40 xcexcmxc3x9740xcexcm, microprocessing at the silicon LSI process level has been already accomplished. Therefore, it is difficult to improve the sensitivity further by devising improved layouts of the support structures. Similarly, it is difficult to further reduce the coefficient of thermal conduction which is one of characteristics of the material of the support structures. Indeed, with respect to conductive interconnect for sending out an electrical signal from the infrared detection portion, two conflicting requirements are imposed, i.e. electrical conduction and thermal conduction which are similar in mechanism. Consequently, it will be difficult to improve sensitivity by further material improvement.
Another method for improving the sensitivity is to improve the ratio of the variation dS in the electrical signal produced by a thermoelectric converter to the variation dTd in the temperature of the infrared detection portion, i.e., dS/dTd. This method is an electrical method. It is important in this method, unlike the other two methods, that various electrical noises produced simultaneously be reduced. Various thermoelectric converter means have been reported.
For example, thermopiles for converting a temperature difference into an electric potential by the Seebeck effect (e.g., Toshio Kanno, et al., Proc. SPIE Vol. 2269, pp. 450-459, 1994), bolometers for converting a temperature change into a resistance change by a temperature variation of a resistor (e.g., A. Wood, Proc. IEDM, pp. 175-177, 1993), pyroelectric devices for converting temperature variations into electric charge by the pyroelectric effect (e.g., Charles Hanson, et al., Proc. SPIE Vol. 2020, pp. 330-339, 1993), and a silicon pn junction for converting a temperature change into a voltage change by a constant forward electric current (e.g., Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999) have been reported.
Of these devices, the infrared detection device making use of a silicon pn junction is described in further detail in FIG. 23 which is a perspective view showing the structure of infrared detection pixels using the lateral pn junction. As shown in FIG. 23, a silicon layer pattern 609 is formed on a laminate structure comprising a silicon substrate 607 and a dielectric film 608. A pn junction is formed in each silicon layer pattern 609. Conductive interconnects 617 are formed between the silicon layer patterns 609 to connect the pn junctions of the silicon layer patterns 609 in series. This structure can obtain a larger voltage change owing to the series connection of the pn junctions. Hence, the detection sensitivity can be improved.
However, the actual situation is that any one electrical detection method is not decisively superior to other methods when these various methods are compared in terms of thermoelectric conversion characteristics, noise characteristics, and fabrication method. For example, bolometers are superior in temperature resolution. Meanwhile, silicon pn junctions are superior in fabrication method, because they can be manufactured only by silicon LSI fabrication steps.
In the formation of infrared detection devices, a unique conductive or metallization layer made of a material of low coefficient of thermal conduction is formed for the support conductive interconnects inside the support structures in order to read out signals from the infrared detecting portion. For example, support conductive structures of titanium material are known.
Where titanium itself is used, a process step for forming only the support conductive interconnect structures is necessary, besides process steps for forming conductive interconnects in device peripheral circuitry. Therefore, the process sequence is unavoidably complicated.
In view of the foregoing circumstances, the present invention has been made. It is an object of the present invention to provide a higher-sensitivity, uncooled infrared sensor that can be fabricated at lower cost by a simpler process method. It is another object of the invention to provide this simpler, lower-cost process method.
To solve the foregoing problems, a first embodiment of the present invention provides an infrared sensor, comprising a substrate including a plurality of infrared detection pixels arrayed on a substrate wherein each of the infrared detection pixels has an infrared absorption portion formed over the substrate and absorbing infrared radiation, a thermoelectric converter portion formed over the substrate and converting a temperature change caused by the infrared radiation absorbed by the infrared absorption portion into an electrical signal, and support structures for supporting the thermoelectric converter portion and the infrared absorption portion over the substrate via a separation space, the support structures having conductive interconnect layers for delivering the electrical signal from the thermoelectric converter portion to the substrate, a pixel selection circuit for selecting at least one of the infrared detection pixels which deliver the electrical signal, and an output circuit for outputting the electrical signal delivered from the selected infrared detection pixels via the conductive interconnect layers, at least one of the pixel selection circuit and the output circuit comprising MOS transistors, wherein the conductive interconnect layers include a material the same as gate layers of the MOS transistors and have the same thickness as the gate layers of the MOS transistors.
Preferably, the above-described first embodiment of the invention has the following configurations.
(1) Each of the conductive interconnect layers and the gate layers include a laminate structure having a polysilicon layer and a metal silicide layer.
(2) Each of the conductive interconnect layers and the gate layers include a laminate structure having a polysilicon layer and a metal layer.
(3) The support structures further include first dielectric layers covering side surfaces of the conductive interconnect layers, and the MOS transistors include second dielectric layers covering side surfaces of the gate layers thereof. The first dielectric layers include the same material as the second dielectric layers of the MOS transistors.
A second embodiment of the present invention provides an infrared sensor including, a substrate, a plurality of infrared detection pixels arrayed on a substrate wherein each of the infrared detection pixels has an infrared absorption portion formed over the substrate and absorbing infrared radiation, a thermoelectric converter portion formed over the substrate and converting a temperature change caused by the infrared radiation absorbed by the infrared absorption portion into an electrical signal, and support structures for supporting the thermoelectric converter portion and the infrared absorption portion over the substrate via a separation space, the support structures having conductive interconnect layers for delivering the electrical signal from the thermoelectric converter portion to the substrate and first dielectric layers covering side surfaces of the conductive interconnect layers, a pixel selection circuit for selecting at least one of the infrared detection pixels which delivers the electrical signal, and an output circuit for outputting the electrical signal delivered from the selected infrared detection pixels via the conductive interconnect layers, at least one of the pixel selection circuit and the output circuit comprising MOS transistors, the MOS transistors having second dielectric layers covering side surfaces of gate layers thereof, wherein the first dielectric layers include a material the same as the second dielectric layers of the MOS transistors.
In the second embodiment of the invention described above, the first dielectric layers are preferably formed to cover the side and top surfaces of the conductive interconnect layers of the support structures. Second dielectric layers narrower than the first dielectric layers are formed on at least one of the upper side of the first dielectric layers and the under side of the conductive interconnect layers.
A third embodiment of the present invention provides an infrared sensor including a substrate, a plurality of infrared detection pixels arrayed on a substrate wherein each of the infrared detection pixels has an infrared absorption portion formed over the substrate and absorbing infrared radiation, a thermoelectric converter portion formed over the substrate and converting a temperature change caused by the infrared radiation absorbed by the infrared absorption portion into an electrical signal, and support structures for supporting the thermoelectric converter portion and the infrared absorption portion over the substrate via a separation space wherein the support structures have conductive interconnect layers for delivering the electrical signal from the thermoelectric converter portion to the substrate, first dielectric layers covering side and top surfaces of the conductive interconnect layers, and second dielectric layers formed on at least one of the upper side of the first dielectric layers and the under side of the conductive interconnect layers wherein the widths of the second dielectric layers are smaller than the first dielectric layers, a pixel selection circuit for selecting at least one of the infrared detection pixels which deliver the electrical signal, and an output circuit for outputting the electrical signal delivered from the selected infrared detection pixels via the conductive interconnect layers.
In the second and third embodiments of the invention, the first dielectric layers include silicon nitride, and the second dielectric layers include silicon oxide.
The above-described first through third embodiments of the invention preferably have the following structures.
(1) The above-described substrate includes a single-crystal silicon support substrate, a silicon oxide layer formed on the single-crystal silicon support substrate, and a single-crystal silicon layer formed on the silicon oxide layer. The thermoelectric converter portion includes the single-crystal silicon layer.
(2) The bottom surfaces of the conductive interconnect layers of the support structures are exposed to the aforementioned separation space overlying the substrate.
(3) The above-described thermoelectric converter portion is exposed to the aforementioned space overlying the substrate.
(4) The above-described thermoelectric converter portion is formed in a single-crystal semiconductor layer, and is fabricated by the pn junction between a region of a first conductivity type and a region of a second conductivity type formed in the single-crystal semiconductor layer.
(5) The infrared absorption portion is fabricated by stacking a silicon nitride film on a silicon oxide film.
(6) The second dielectric layer on the first dielectric layer includes a layer the same as the silicon oxide film of the infrared absorption portion.
A fourth embodiment of the present invention provides a method of fabricating an infrared sensor having a plurality of infrared detection pixels on a substrate wherein each of the infrared detection pixels has an infrared absorption portion for absorbing infrared radiation and a thermoelectric converter portion for converting a temperature change caused by the infrared radiation absorbed by the infrared absorption portion into an electrical signal. The method includes forming the thermoelectric converter portion on the substrate, forming an conductive film on the substrate, patterning the conductive film to form first conductive film patterns in first areas where the infrared detection pixels should be formed and second conductive film patterns in second areas other than the first areas, etching portions of the substrate under the thermoelectric converter portions and the first conductive film patterns to form support structures for supporting the thermoelectric converter portions over the substrate via a separation space wherein the support structures have the first conductive film patterns as conductive interconnect layers for delivering the electrical signal from the infrared detection pixels, forming MOS transistors having gate layers including the second conductive film patterns, forming a pixel-selecting circuit for selecting at least one of the infrared detection pixels which deliver the electrical signal, the pixel-selecting circuit including at least one of the MOS transistors, and forming an output circuit for outputting the electrical signal delivered from the selected infrared detection pixels via the conductive interconnect layers wherein the output circuit includes at least one of the MOS transistors.
Preferably, the above-described fourth embodiment of the invention has the following structures.
(1) The conductive film includes a laminate structure including a polysilicon layer and a metal silicide layer, and each of conductive interconnect layers and the gate layers includes the polysilicon/metal silicide laminate structure.
(2) The conductive film includes a laminate structure having a polysilicon layer and a metal layer, and each of conductive interconnect layers and the gate layers includes the polysilicon/metal laminate structure.
(3) The method further includes forming first dielectric layers on side and top surfaces of gate layers of the MOS transistors, and forming second dielectric layers to cover side and top surfaces of the conductive interconnect layers of the support structures, wherein the first and second dielectric layers are formed patterning the same dielectric film.
A fifth embodiment of the present invention provides a method of fabricating an infrared sensor including a plurality of infrared detection pixels on a substrate wherein each of the infrared detection pixels has an infrared absorption portion for absorbing infrared radiation and a thermoelectric converter portion for converting a temperature change caused by the infrared radiation absorbed by the infrared absorption portion into an electrical signal. The method includes forming first conductive film patterns on first areas where the infrared detection pixels should be formed, forming second conductive film patterns on second areas other than the first areas, forming first dielectric layers to cover side and top surfaces of each of the first and second conductive film patterns, etching portions of the substrate under the thermoelectric converter portions and the first conductive film patterns to form support structures for supporting the thermoelectric converter portions over the substrate via separation a space wherein the support structures have the first conductive film patterns as conductive interconnect layers for delivering the electrical signal from the infrared detection pixels, forming MOS transistors having gate layers including the second conductive film patterns and the first dielectric layers, forming a pixel-selecting circuit for selecting at least one of the infrared detection pixels which deliver the electrical signal wherein the pixel-selecting circuit include at least one of the MOS transistors, and forming an output circuit for outputting the electrical signal delivered from the selected infrared detection pixels via the conductive interconnect layers wherein the output circuit includes at least one of the MOS transistors.
A sixth embodiment of the present invention provides a method of fabricating an infrared sensor including a plurality of infrared detection pixels on a substrate wherein each of the infrared detection pixels has an infrared absorption portion for absorbing infrared radiation and a thermoelectric converter portion for converting a temperature change caused by the infrared radiation absorbed by the infrared absorption portion into an electrical signal. The method includes forming recessed portions in areas of the substrate where the infrared detection pixels should be formed, forming isolation dielectric layers in the recessed portions, forming first conductive film patterns on the isolation dielectric layers, forming a first dielectric layers to cover side and top surfaces of the first conductive film patterns, forming a second dielectric layer on the first dielectric layers, etching portions of the substrate under the thermoelectric converter portions and the first conductive film patterns to form support structures for supporting the thermoelectric converter portions over the substrate via a separation space wherein the support structures have the first conductive film patterns as conductive interconnect layers for delivering the electrical signal from the infrared detection pixels, selectively etching the isolation dielectric layers and the second dielectric film relative to the first dielectric layers to remove at least one of the isolation dielectric layers and portions of the second dielectric film on the first dielectric layers or to make widths thereof smaller than the first dielectric layers, forming a pixel-selecting circuit for selecting at least one of the infrared detection pixels which deliver the electrical signal wherein the pixel-selecting circuit includes at least one MOS transistor, and forming an output circuit for outputting the electrical signal delivered from the selected infrared detection pixels via the conductive interconnect layers wherein the output circuit includes at least one MOS transistor.
The fifth and sixth embodiments of the present invention described thus far preferably have the following structures.
(1) The first dielectric layers include silicon nitride, and the second dielectric layers include silicon oxide.
(2) A liquid mixture of acetic acid and ammonium fluoride is used as an etchant for etching the silicon oxide layer which is the second dielectric layer.
The above-described fourth through sixth embodiments of the present invention preferably have the following structures.
(1) The aforementioned substrate includes a single-crystal silicon support substrate, a silicon oxide film formed on this support substrate, and a single-crystal silicon layer formed on the silicon oxide film. The above-described thermoelectric converter portions are formed in the single-crystal silicon layer.
(2) The thermoelectric converter portions are fabricated by forming regions of first and second conductivity types, respectively, on the single-crystal semiconductor layer so as to form pn junctions.
(3) The infrared absorption portions are fabricated by stacking a silicon nitride film on the silicon oxide film.
(4) The second dielectric film includes a layer the same as the silicon oxide film of the infrared absorption portions.
(5) The single-crystal support substrate is a single-crystal silicon substrate. During forming the support structures, an etchant that wet etches the single-crystal silicon anisotropically is used as the etchant for etching the single-crystal support substrate.
According to the present invention, if a layout on a plane limited by a level of micromachining remains the same, then the cross-sectional area of the support structures between the infrared detection portions and the support substrate can be reduced. Therefore, heat transfer through the support structures, dominating the heat transportation between each infrared radiation detection portion and the support substrate, can be reduced. As a result, a high-sensitivity, uncooled infrared sensor can be obtained.
Furthermore, according to the present invention, the width of the support structures is reduced to decrease the cross-sectional area of the support structures. In spite of the decrease in the cross-sectional area of the support structures, the mechanical strength necessary for the support of the infrared sensor portions does not decrease. Also, it is unlikely that the acceleration resistance decreases.
In addition, according to the present invention, the bottom surfaces of the infrared detection portions and parts or bottom surfaces of the support conductive interconnects are exposed. The material of the infrared detection portions (e.g., a single-crystal silicon) and a metal material (e.g., titanium) or polysilicon forming the support conductive interconnects are much lower in infrared emissivity in the 10 xcexcm-band than the prior art diaphragm structure and the silicon oxide film or silicon nitride film existing at the bottoms of the support structures. Accordingly, with the bottom-exposed structures described above, heat transportation due to radiation from the bottom surfaces can be reduced. As micromachining technology progresses, pixels and support structures will become finer. Under this trend, it is forecasted that heat transportation due to radiation from the silicon oxide film or silicon nitride film existing at the bottoms of the aforementioned diaphragm structures and support structures will be at the same level as the heat conduction through the support structures. Where the sensitivity is increased simply by reductions in dimensions, heat transportation owing to the aforementioned radiation will create a sensitivity limitation. Accordingly, under the trend toward miniaturization, the above-described bottom-exposed structures can provide a higher-sensitivity, uncooled infrared sensor.
Further, according to the present invention, the support conductive interconnects formed inside the support structures for readout of signals from the infrared detection portions are formed from a layer the same as the gate electrodes of MOS transistors formed in device peripheral circuitry. Therefore, the unique support conductive interconnect layer that has been necessary conventionally is dispensed with. In consequence, the number of process steps can be decreased. Additionally, the device fabrication yield can be improved. Hence, a low-cost, uncooled infrared sensor can be obtained. Moreover, the support conductive interconnect structures can be made finer, because the support conductive interconnects are made of a layer the same as the gate electrodes of the MOS transistors described above. Further, use of a structure of polycide or polymetal that has a low resistivity makes it possible to obtain high-sensitivity device characteristics.
Further, according to the present invention, a first dielectric film is formed on the support conductive interconnects, and the support conductive interconnects can be protected against etching by the dielectric film. Accordingly, where the substrate or the second dielectric film (device-isolating dielectric film, interlayer dielectric film on the substrate, buried dielectric film of SOI, or the like) formed on top of or under the support conductive interconnects is etched, the support conductive interconnects can be protected against the etching. This can prevent various problems including excessive thinning of the support conductive interconnects, which would otherwise lead to an increase of the resistivity and generation of breakage of the support conductive interconnects, thus causing defects.
Further, the aforementioned first dielectric film can be built simultaneously with the gate sidewall dielectric film formed on the sidewalls of the gates of the MOS transistors in the peripheral circuitry. Specifically, where a dielectric film such as a silicon nitride film is formed on the sidewalls of the gates of the MOS transistors in the peripheral circuitry and the dielectric film is etched, the dielectric film may be selectively left on the gate sidewalls. A second dielectric film can be formed simultaneously (in the same layer) with the step of formation of the first-mentioned dielectric film on the support conductive interconnects. Also, in this case, the number of process steps can be decreased. Especially, by employing both the step of formation of this dielectric film and the step of forming the gate electrodes of the MOS transistors in the peripheral circuitry in the same layer, high-sensitivity support structures can be manufactured at low cost and at high yield by making maximum use of the process matching.
As described thus far, the present invention makes it possible to obtain a low-cost, high-sensitivity, uncooled infrared sensor.